The present invention relates to a marker for a navigation system.
Markers are used, for example in image-assisted surgical procedures (IGS=Image Guided Surgery), for ascertaining the position of surgical instruments or bodies connected to, for example, a plurality of markers in the form of a reference star, by means of optical detection of the light reflected by the markers, and, on the basis of the ascertained positional information, carrying out a surgical intervention.
Surgical navigation systems are accordingly used for measuring geometric variables (usually in a sterile context) in the course of surgical interventions on human beings and animals. A much used measurement technique is, as already mentioned, the positional measurement of optical markers using a light-optical camera system. Either these optical markers are self-illuminating (=active markers) or they reflect light produced by a flash-illumination camera system (=passive markers). On account of the particular conditions, especially in the sterile context of a surgical intervention, passive elements have advantages because they do not require a power supply of their own. In a process of sterile re-processing (washing in a washing machine, or sterilisation at 134° C. in steam), an energy supply and illuminating elements place very high demands on the resilience and construction of such components.
In the case of the passive markers, the light shining along the optical axis of the measuring objective, which light is usually produced by annular flash lamps arranged surrounding the camera objective, is reflected in the opposite direction (=retroreflection). Because the positional measurement of a surgical instrument or of the geometry of a body to be measured should be independent of the viewing angle, the use of spherical markers represents the simplest and most reliable solution from a measurement technique point of view. Irrespective of the viewing direction, a sphere always projects to a circle, whereas flat geometries, such as disc-shaped markers, produce distorted images or contours, for example contours similar to ellipses, which can be evaluated only with relatively great difficulty. Depending on the application, the diameter of the marker spheres preferably used is between 5 mm and 20 mm.
Retroreflection is achieved by means of a specially formed surface. Coatings having very fine prisms or coatings having very fine glass spheres embedded in the surface are known. Typical dimensions of those very fine glass spherules are between 20μ and 200μ. Given a suitable refractive index relative to the surrounding air and given suitable reflection properties of the embedding surface, which in the case of glass spherules represents a miniaturised hollow mirror, the incident light is retroreflected. Such surfaces are widely used in security technology and are often produced in foil form.
Because of the fineness of the glass spherules, a retroreflecting surface is rough at a micro scale. If such a retroreflecting surface is covered with a substance which is not transparent, for example dirt particles or blood, this region appears as a dark spot in the imaging by the camera. Because of the roughness at a micro scale, the removal of such soiling is laborious and also very often incomplete because dirt remains behind in the gaps between the glass spherules. If such a surface is wetted with a transparent medium, for example water, the surface loses its retroreflecting property because of the disrupted “air-glass spherule” beam path. In the image of the measuring camera, a retroreflecting surface wetted with water appears dark and cannot be used for measurement.
In respect of the formation of retroreflecting marker spheres, reference is made additionally to FR 2 706 045 A1. As far as the use of such marker spheres in surgery is concerned, reference is made to WO 2007/090288 A1.